This invention relates in general to a rotor for a rotary electrical machine having a superconductive field winding. More particularly, it relates to a rotor for a rotary electrical machine having a superconductive field winding, wherein the transfer of heat from a damper disposed for preventing a varying magnetic field from being applied to the superconductive field winding to the superconductive field winding is reduced.
A superconductive rotary electrical machine generally employs a superconductive wire for the field winding of the rotor thereof. The superconductive wire is made of a metallic material whose electric resistance becomes zero when it is cooled to an extremely low temperature close to the absolute zero point. The magnetic field which is generated by a field winding of superconductive wire can be made more intense than that provided in a conventional rotary electrical machine, and hence, as a result of use of a superconductive field winding, the output of the rotary electrical machine can be intensified. However, when the superconductive wire is subjected to a varying magnetic field of significantly great magnitude, it becomes heated and therefore becomes incapable of maintaining the superconducting state with the result that it transfers to the normally conducting state. For this reason, it is conventional to provide a damper made of a good electrical conductor, such as copper and aluminum, to cover the superconductive field winding and thereby shield it from varying magnetic fields. This is disclosed in and is known from U.S. Pat. No. 3,679,920.
As stated previously, the superconductive wire produces its beneficial advantages only if it operates in the superconducting state when cooled to an extremely low temperature. It is, therefore, indispensable to maintain the superconductive wire at the extremely low temperature required for superconductivity. However, the damper functions as a shield so that, where a magnetic flux changes (as at the time when the rotary electrical machine becomes out of order) an eddy current is caused to flow in the damper thereby to prevent the magnetic flux change from being transmitted to the superconductive wire inside the damper. Joule loss is generated in the damper by the eddy current, and the damper is thereby heated. The Joule loss is very great, and a temperature rise to above 200.degree. C., which can result in undesirable transfer of heat to the superconducting field winding, occurs locally.
In order to resist the electromagnetic force acting on the damper, a cylindrical damper support is disposed in contact with the inner surface of the damper. Due to the heating of the damper, the damper support is also heated, and radiant heat from the damper support is transferred to the superconductive field winding. When a large amount of radiant heat is transferred to the superconductive field winding, the temperature of the superconductive wire is raised, and the superconductive wire becomes incapable of maintaining the superconducting state.
In order to solve this problem, it has been proposed to cool the damper by providing cooling passage holes in the damper and causing a cooling medium to flow therethrough. However, the efficiency of cooling with a coolant is limited, and, therefore, it is impossible to fully remove the heat generated in the damper by the use of the coolant.